High speed transportation, in particular on highways has created a demand for measuring surface friction between vehicle tires and the roadway.
Such a method is known from U.S. Pat. No. 45 45 240 to Leiber. In the known arrangement an anti-locking system which is provided for different reasons is used for automatically applying increasing test pressure to the brakes for determining the slip between a tire and a roadway. When a predetermined value of slip below a given test pressure value is reached, an alarm signal is emitted.
In the case of the arrangement known from Leiber, actual slip is compared with only one set value, and the display tells the driver whether the measured slip is greater or smaller than the set value. There is not continuous measuring and indication of the coefficient of friction between a tire and a roadway.
In U.S. Pat. No. 38 93 330 to Shute et al. an apparatus for measuring coefficients of friction between a tire of a braked vehicle wheel and a road surface is described which utilizes a special fifth wheel in addition to the normal four wheels of the vehicle. The coefficient of friction is determined by measuring both the speed of the vehicle at the time of brake engagement and the distance traveled by the vehicle after brake engagement. The total stopping distance of the vehicle as well as vehicle speed and distance are measured from which the coefficient of friction can be calculated.
In U.S. Pat. No. 42 12 063 to Hardmark a method for determining the coefficient of friction between a vehicle tire and a roadway is described in which, in addition to the normal wheels of the vehicle, an additional measuring wheel is required. The measuring wheel is pivotable mounted in the vehicle in a vertical plane and can be moved into contact with the roadway. The vertical forces acting on the measuring wheel are pre-programmed with a constant factor and the coefficient of friction is determined from the measured value of the tensional force acting on the measuring wheel. Said tensional force corresponds to the torque acting on the measuring wheel and, since the vertical load on the measuring wheel is programmed in as a constant factor, the signal represents the coefficient of friction between the tire and the roadway. The coefficient is not determined on the basis of measuring slip between the measuring wheel and another wheel.